Diffusion

· The effect of the diffusion process shown is to change the chemical composition of the zone of interpenetration. Thermodynamic arguments indicate that the presence of impurity atoms in a matrix can reduce its Gibbs function.
· For interstitial impurities, the equilibrium concentration, n_I ,depends upon their Enthalpy of formation, H_I ,and the number of interstitial sites, M_I,
X_I = (n_I / M_I ) = exp (- H_I / kT)
· Similarly, the expression for the equilibrium concentration of substitutional impurities has the form:
X_B= (n_B/N) ~ exp (- H_B / kT)
where H_B = Enthalpy of solution of a B-atom in an A-atom matrix with, n_B, B-atoms and (N - n_B) A-atoms for a total of N lattice sites.